1. Field of the Invention
This invention relates to high density polyethylene resins having improved processing stability.
2. Description of the Prior Art
High density polyethylene (HDPE) is a well-known and highly useful thermoplastic resin recognized for its excellent physical properties and chemical resistance. HDPEs are linear homopolymers and copolymers of ethylene having densities in the range 0.94 to 0.965 g/cc. Molecular weights typically range from about 50,000 up to about 500,000; however, resins with molecular weights up to several million, referred to as ultra high molecular weight HDPEs, have been produced.
The ability to vary the density, molecular weight and molecular weight distribution of HDPE resins makes them a highly versatile material suitable for use in a wide variety of diverse applications. For example, HDPEs can be used for extrusion and powder coating, to produce blown or cast films, for rotational molding and rotational lining, for injection molding, and for blow molding.
Because polyolefins are susceptible to oxidation at various stages in the life cycle of the resin, i.e., during manufacture, processing and end use, antioxidants are needed to protect against the deleterious effects of oxygen and temperature and preserve the inherent properties of the resin. Primary stabilization is generally achieved by incorporating one or more sterically hindered phenols which function as radical scavengers in the resin. Secondary antioxidants, or processing stabilizers as they are often called, are also often required. The secondary antioxidants provide a complementary protection mechanism against peroxides and hydroperoxides that would otherwise react in a detrimental way with the resin and produce undesirable changes in melt viscosity and color formation. Phosphorus compounds, e.g., phosphites and phosphonites, are commonly used as secondary antioxidants with sterically hindered phenols for the stabilization of polyolefins.
Chain scission and chain extension, i.e., crosslinking, occur simultaneously during HDPE processing depending on the availability of oxygen, type and amount of catalyst residue, and processing conditions. Polymer type also plays a significant role when considering changes which can occur during processing. Phillips' process HDPEs have significantly higher terminal vinyl unsaturation content than Ziegler-Natta resins and, therefore, are notably more prone to increases in melt viscosity (reduction in melt index) and formation of insoluble gel particles due to chain extension during processing. The presence of these insoluble polymer molecules can lead to the formation of undesirable defects, including "black specks," in films or blow molded articles produced from the resin.
While proper choice of the secondary antioxidant is essential if the aforementioned processing problems are to be avoided, selection is complicated in that processing stabilizers exhibit different degrees of effectiveness for HDPEs produced by the different processes. To illustrate this point, reference may be made to the table provided at page 33 of Plastics Additives, 2nd edition, R. Gachter and H. Muller (1987) ranking the relative effectiveness of various antioxidants for Ziegler and Phillips resins.
Various phosphites and phosphonites have been evaluated as secondary antioxidants for HDPE and the results are reported in the literature. In a study conducted by F. Mitterhofer and reported at pp. 809-826 in Science and Technology of Polymer Processing, Proceedings of the International Conference on Polymer Processing held at The Massachusetts Institute of Technology, Cambridge, Mass., August, 1977, the author compared the effectiveness of distearyl-pentaerythritol-diphosphite (P-1) and tetrakis (2,4-di- tert.butylphenyl)4,4'-biphenylenediphosphonite (PEPQ) in Phillips' process HDPE. By measuring the melt index change during prolonged residence in a melt index apparatus, it was concluded that only PEPQ at higher concentrations (2500 ppm) gave no change in melt flow. In all instances when PEPQ was used at lower concentrations with a hindered phenol, a lowering of melt index was observed - the extent of the reduction varying with the weight ratio of PEPQ to hindered phenol. A later article published by the same author (Polymer Eng. & Sci., mid-July, 1980, Vol. 20, No. 10, pp. 692-695) reported results evaluating PEPQ in a low melt index Phillips' process HDPE resin using a Brabender plastograph and noting the time to the onset of crosslinking, i.e., a marked increase in torque. Procedures of this type using torque rheometers provide a more rigorous test, i.e., higher shear rates, than can be achieved with the melt index apparatus and are generally considered to give good correlation with actual processing conditions. At best, under these more rigorous test conditions with PEPQ, it was only possible to delay the onset of crosslinking.
It would be highly advantageous if the significant torque rise heretofore observed during processing of Phillips process HDPE in torque rheometers, and attributable to the onset of undesirable levels of crosslinking, could be eliminated. It would be even more desirable if elimination of the abrupt torque rise could be achieved using significantly lower levels of processing stabilizer than heretofore reported in the prior art. It would be still more advantageous if the above-noted improvements in processability, i.e., ability to eliminate significant and abrupt changes in melt viscosity of the HDPE during extended processing, could be achieved using a known phosphite compound with conventional hindered phenols.